For industrial applications the measurement of the flow rate of gases or liquids in tubing systems is of big importance. Most of the existing measurement systems utilize the liquid or gas flow to drive a wheel with its number of turns per unit time being proportional to the flow rate. These systems however lack reliability over longer time periods.
Other known measurement systems apply ultrasound. Here the phase shift of an ultrasonic wave introduced by a flowing medium is a measure of the flow velocity of the medium. Usually an ultrasonic transmitter and an ultrasonic receiver are mounted in a tubing system with a given angle with respect to the tube axis. A corresponding example is described in EP 0 040 837 A1. Such systems have a rather complex geometry and suffer from the additional disadvantage that the measuring accuracy will be deteriorated because the phase shift depends on the cosine of the given angle which is different from 0 degrees.
Principally the flow velocity of a medium may as well be determined from the time delay of ultrasound from an ultrasonic transmitter to an ultrasonic receiver where the transmitter is operated in the pulsed mode.